[syzbot] [overlayfs?] possible deadlock in mnt_want_write (8)

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: ec7ef10b77a6 Merge branch 'for-next/core' into for-kernelci
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
console output: https://syzkaller.appspot.com/x/log.txt?x=17a17dd2580000
kernel config: https://syzkaller.appspot.com/x/.config?x=522222e78af9b9d2
dashboard link: https://syzkaller.appspot.com/bug?extid=1e5626d6fd59853cbfaa
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
userspace arch: arm64

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/22c81873d970/disk-ec7ef10b.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ba2c817c4ea9/vmlinux-ec7ef10b.xz
kernel image: https://storage.googleapis.com/syzbot-assets/04c4e1c1bb56/Image-ec7ef10b.gz.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+1e5626...@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Tainted: G L
------------------------------------------------------
syz.5.673/7842 is trying to acquire lock:
ffff0000d814a410 (sb_writers#3){.+.+}-{0:0}, at: mnt_want_write+0x44/0x9c fs/namespace.c:493

but task is already holding lock:
ffff0000dc947230 (&ovl_i_lock_key[depth]){+.+.}-{4:4}, at: ovl_inode_lock_interruptible fs/overlayfs/overlayfs.h:705 [inline]
ffff0000dc947230 (&ovl_i_lock_key[depth]){+.+.}-{4:4}, at: ovl_nlink_start+0xc8/0x350 fs/overlayfs/util.c:1173

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&ovl_i_lock_key[depth]){+.+.}-{4:4}:
__mutex_lock_common kernel/locking/mutex.c:646 [inline]
__mutex_lock+0x160/0xef8 kernel/locking/mutex.c:820
mutex_lock_interruptible_nested+0x24/0x30 kernel/locking/mutex.c:898
ovl_inode_lock_interruptible fs/overlayfs/overlayfs.h:705 [inline]
ovl_copy_up_start+0x58/0x264 fs/overlayfs/util.c:735
ovl_copy_up_one fs/overlayfs/copy_up.c:1182 [inline]
ovl_copy_up_flags+0x768/0x28a4 fs/overlayfs/copy_up.c:1243
ovl_copy_up+0x24/0x34 fs/overlayfs/copy_up.c:1282
ovl_link+0x8c/0x26c fs/overlayfs/dir.c:774
vfs_link+0x40c/0x580 fs/namei.c:5774
filename_linkat+0x224/0x478 fs/namei.c:5842
__do_sys_linkat fs/namei.c:5868 [inline]
__se_sys_linkat fs/namei.c:5863 [inline]
__arm64_sys_linkat+0xe8/0x118 fs/namei.c:5863
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
el0_svc+0x64/0x260 arch/arm64/kernel/entry-common.c:736
el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:755
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594

-> #1 (&ovl_i_mutex_key[depth]){+.+.}-{4:4}:
down_write+0x50/0xc0 kernel/locking/rwsem.c:1625
inode_lock include/linux/fs.h:1029 [inline]
lock_two_nondirectories+0xc4/0x148 fs/inode.c:1254
ext4_move_extents+0x194/0x3580 fs/ext4/move_extent.c:589
__ext4_ioctl fs/ext4/ioctl.c:1657 [inline]
ext4_ioctl+0x2a14/0x4234 fs/ext4/ioctl.c:1922
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:597 [inline]
__se_sys_ioctl fs/ioctl.c:583 [inline]
__arm64_sys_ioctl+0x14c/0x1c4 fs/ioctl.c:583
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
el0_svc+0x64/0x260 arch/arm64/kernel/entry-common.c:736
el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:755
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594

-> #0 (sb_writers#3){.+.+}-{0:0}:
check_prev_add kernel/locking/lockdep.c:3165 [inline]
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x1780/0x2f44 kernel/locking/lockdep.c:5237
lock_acquire+0x140/0x368 kernel/locking/lockdep.c:5868
percpu_down_read_internal include/linux/percpu-rwsem.h:53 [inline]
percpu_down_read_freezable include/linux/percpu-rwsem.h:83 [inline]
__sb_start_write include/linux/fs/super.h:19 [inline]
sb_start_write+0x5c/0x294 include/linux/fs/super.h:125
mnt_want_write+0x44/0x9c fs/namespace.c:493
ovl_want_write fs/overlayfs/util.c:38 [inline]
ovl_nlink_start+0x154/0x350 fs/overlayfs/util.c:1177
ovl_do_remove+0x1c0/0xa04 fs/overlayfs/dir.c:954
ovl_rmdir+0x24/0x34 fs/overlayfs/dir.c:993
vfs_rmdir+0x34c/0x5a0 fs/namei.c:5366
filename_rmdir+0x208/0x3dc fs/namei.c:5421
__do_sys_unlinkat fs/namei.c:5596 [inline]
__se_sys_unlinkat fs/namei.c:5589 [inline]
__arm64_sys_unlinkat+0xc8/0x104 fs/namei.c:5589
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
el0_svc+0x64/0x260 arch/arm64/kernel/entry-common.c:736
el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:755
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594

other info that might help us debug this:

Chain exists of:
sb_writers#3 --> &ovl_i_mutex_key[depth] --> &ovl_i_lock_key[depth]

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
lock(&ovl_i_lock_key[depth]);
lock(&ovl_i_mutex_key[depth]);
lock(&ovl_i_lock_key[depth]);
rlock(sb_writers#3);

*** DEADLOCK ***

4 locks held by syz.5.673/7842:
#0: ffff0000d924c410 (sb_writers#17){.+.+}-{0:0}, at: mnt_want_write+0x44/0x9c fs/namespace.c:493
#1: ffff0000dc944300 (&ovl_i_mutex_dir_key[depth]/1){+.+.}-{4:4}, at: inode_lock_nested include/linux/fs.h:1074 [inline]
#1: ffff0000dc944300 (&ovl_i_mutex_dir_key[depth]/1){+.+.}-{4:4}, at: __start_dirop+0xa0/0xf4 fs/namei.c:2914
#2: ffff0000dc946ec0 (&ovl_i_mutex_dir_key[depth]){++++}-{4:4}, at: inode_lock include/linux/fs.h:1029 [inline]
#2: ffff0000dc946ec0 (&ovl_i_mutex_dir_key[depth]){++++}-{4:4}, at: vfs_rmdir+0x110/0x5a0 fs/namei.c:5351
#3: ffff0000dc947230 (&ovl_i_lock_key[depth]){+.+.}-{4:4}, at: ovl_inode_lock_interruptible fs/overlayfs/overlayfs.h:705 [inline]
#3: ffff0000dc947230 (&ovl_i_lock_key[depth]){+.+.}-{4:4}, at: ovl_nlink_start+0xc8/0x350 fs/overlayfs/util.c:1173

stack backtrace:
CPU: 1 UID: 0 PID: 7842 Comm: syz.5.673 Tainted: G L syzkaller #0 PREEMPT
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Call trace:
show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:499 (C)
__dump_stack+0x30/0x40 lib/dump_stack.c:94
dump_stack_lvl+0xd8/0x12c lib/dump_stack.c:120
dump_stack+0x1c/0x28 lib/dump_stack.c:129
print_circular_bug+0x328/0x330 kernel/locking/lockdep.c:2043
check_noncircular+0x158/0x174 kernel/locking/lockdep.c:2175
check_prev_add kernel/locking/lockdep.c:3165 [inline]
check_prevs_add kernel/locking/lockdep.c:3284 [inline]
validate_chain kernel/locking/lockdep.c:3908 [inline]
__lock_acquire+0x1780/0x2f44 kernel/locking/lockdep.c:5237
lock_acquire+0x140/0x368 kernel/locking/lockdep.c:5868
percpu_down_read_internal include/linux/percpu-rwsem.h:53 [inline]
percpu_down_read_freezable include/linux/percpu-rwsem.h:83 [inline]
__sb_start_write include/linux/fs/super.h:19 [inline]
sb_start_write+0x5c/0x294 include/linux/fs/super.h:125
mnt_want_write+0x44/0x9c fs/namespace.c:493
ovl_want_write fs/overlayfs/util.c:38 [inline]
ovl_nlink_start+0x154/0x350 fs/overlayfs/util.c:1177
ovl_do_remove+0x1c0/0xa04 fs/overlayfs/dir.c:954
ovl_rmdir+0x24/0x34 fs/overlayfs/dir.c:993
vfs_rmdir+0x34c/0x5a0 fs/namei.c:5366
filename_rmdir+0x208/0x3dc fs/namei.c:5421
__do_sys_unlinkat fs/namei.c:5596 [inline]
__se_sys_unlinkat fs/namei.c:5589 [inline]
__arm64_sys_unlinkat+0xc8/0x104 fs/namei.c:5589
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
el0_svc+0x64/0x260 arch/arm64/kernel/entry-common.c:736
el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:755
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594


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